The present disclosure relates to a technology for detecting the toner concentration of a two-component developer in which a carrier formed of a magnetic material and a toner are mixed.
A toner concentration sensor is known that detects a toner concentration by utilizing the fact that when the ratio of a toner to a carrier in a two-component developer is changed, the resonant frequency of an LC resonant circuit including a detection coil is changed. For example, a toner concentration sensor has already been proposed in which since the resonant frequency of an LC resonant circuit is changed, and thus the pulse number of a pulse signal output from the LC resonant circuit is changed, the pulse number is counted, with the result that the toner concentration is detected.